The incorporation of particles from millimeter-scale down to nanometers in size is ubiquitous in end-use products produced in industrial-scale quantities. A significant percentage of the particles used across all industries require that the surfaces be coated with a shell, layer, film, or other coating, ranging from sub-nanometer to hundreds of micrometers in thickness. For a variety of reasons, each sector or industry has determined that the incorporation of coated particles into the end-use product provides enough value-add in the performance of the product that the cost associated with each coating process is justified. Vapor deposition techniques are sometimes used to deposit the coatings. Examples of vapor deposition techniques can include chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), molecular layer deposition (MLD), vapor phase epitaxy (VPE), atomic layer chemical vapor deposition (ALCVD), ion implantation or similar techniques. In each of these, coatings are formed by exposing the powder to reactive precursors, which react either in the vapor phase (in the case of CVD, for example) or at the surface of the powder particles (as in ALD and MLD). These processes can be augmented by the incorporation of plasma, pulsed or non-pulsed lasers, RF energy, and electrical arc or similar discharge techniques.
Vapor deposition processes are usually operated batch-wise in reaction vessels such as fluidized bed reactors, rotary reactors and V-blenders, amongst others. Batch processes have significant inefficiencies when operated at large scale, for several reasons. The reactor throughput is a function of the total particle mass or volume loaded into a certain sized vessel for a given process, the total process time (up-time), and the total time between processes (down-time) to load, unload, clean, prepare, etc. Batch processes incur large down-times because at the end of each batch the finished product must be removed from the reaction equipment and fresh starting materials must be charged to the equipment before the subsequent batch can be produced. Equipment failures and maintenance add to this downtime. Process equipment tends to be very large and expensive in batch processes. The need to operate these processes under vacuum adds greatly to equipment costs, especially as equipment size increases. Because of this, equipment costs for batch processes tend to increase faster than operating capacity. Another problem that occurs as the process equipment becomes larger is that it becomes more difficult to maintain uniform reaction conditions throughout the vessel. For example, temperatures can vary considerably within a large reaction vessel. It is also difficult to adequately fluidize a large mass of particles, specifically nanoparticles. Issues such as these can lead to inconsistencies and defects in the coated product.
In vapor deposition processes such as ALD and MLD, the particles are contacted with two or more different reactants in a sequential manner. This represents yet another problem for a batch operation. For a traditional batch process, all cycles are performed sequentially in a single reaction vessel. The batch particle ALD process inherently incurs additional down-time due to more frequent periodic cleaning requirements, and the reaction vessels cannot be used for multiple film types when cross-contamination could be problematic. In addition, the two sequential self-limiting reactions may occur at different temperatures, requiring heating or cooling of the reactor between cycle steps in order to accommodate each step. The throughput for a batch process can be increased either by building larger reaction vessels and/or operating identical reaction vessels in parallel. The capital cost-effective tendency to counteract this down-time from a throughput perspective is to build a larger reaction vessel. With larger vessels, localized process conditions, including internal bed heating, pressure gradients, mechanical agitation to break up nanoparticle aggregates, and diffusion limitations amongst others, become more difficult to control. There is a practical maximum reaction vessel size when performing ALD processes on fine and ultra-fine particles, which limits the annual throughput for a single batch reactor operating continually, where the time duration of the process producing a given amount of coated materials equals the up-time plus down-time. There is a practical maximum allowable capital expense to fabricate a particle ALD production facility, which effectively limits the number of batch reactors that operate identical processes in parallel. With these constraints, there are practical throughput limitations that prohibit the integration of some particle ALD processes at the industrial scale. There is a need to develop a high throughput semi-continuous or continuous-flow ALD process in order to meet industrial scale demands.
In vapor deposition processes such as CVD, the particles can be contacted with two or more different reactants concurrently, or by one or more reactants that do not exhibit the self-limiting behavior characteristic of ALD and MLD processes. For a traditional batch CVD process, the primary methods of controlling reactions are limited to reactant exposure time and operating conditions such as process temperatures and pressures. The batch particle CVD process inherently has limited opportunity to prevent unwanted gas-phase side reactions. There is also a practical maximum reaction vessel size when performing batch particle CVD processes as small variations in the process conditions can lead to large variations in product quality throughout the batch of particles produced. There is a need to develop a high throughput semi-batch or semi-continuous particle CVD process in order to meet industrial scale demands without sacrificing product quality.